Reboilers are sometimes inserted into the bottom of a tower. These are called “stab-in” reboilers. It is not a terribly good idea, because it makes it more difficult to fix a leaking or fouled reboiler without opening the tower itself. However, the “kettle” reboiler, shown in Fig. 7.7, has essentially the same process performance characteristics as the stab-in reboiler, but is entirely external to the tower.
Note that in a kettle reboiler the bottoms product level control valve does not control the level in the tower; it controls the level on the product side of the reboiler only. The liquid level on the boiling or heat-exchanger side of the kettle is controlled by the internal overflow baffle. But what controls the tower-bottom liquid level?
To answer this, let us see how such a gravity-fed or kettle reboiler works:
1. Liquid flows out of the tower into the bottom of the reboiler’s shell.
2. The liquid is partially vaporized.
3. The domed top section of the reboiler separates the vapor and the liquid.
4. The vapor flows back to the tower through the riser line. This is the column’s stripping vapor or heat source.
5. The liquid overflows the baffle. The baffle is set high enough to keep the tubes submerged. This liquid is the bottoms product.
The liquid level in the bottom of the tower is the sum of the following factors:
• The nozzle exit loss of the liquid leaving the bottom of the tower
• The liquid feed-line pressure drop
• The shell-side exchanger pressure drop, which includes the effect of the baffle height
• The vapor-line riser pressure drop, including the vapor outlet nozzle loss
Note that it is the elevation, or the static head pressure, in the tower that drives the kettle reboiler. That is why we call it a gravityfed reboiler. Also, the pressure in the kettle will always be higher than the pressure in the tower. This means that an increase in the reboiler heat duty results in an increase of liquid level in the bottom of the tower.
Should the liquid level in the bottom of the tower rise to the reboiler vapor return nozzle, the tower will certainly flood, but the reboiler heat duty will continue. Unfortunately, reboiler shellside fouling may also lead to tray flooding. This happens because the fouling can cause a pressure-drop buildup on the shell side of the reboiler.
Remember, though, that the increased tower-bottom liquid level will not be reflected on the indicated bottom level seen in the control room, which is actually the level at the end of the kettle reboiler. This is a constant source of confusion to many operators who have towers that flood as a result of high liquid levels, yet their indicated liquid level remains normal.